Photocopier exposure control

ABSTRACT

An automatic exposure control for a reflex photocopier wherein a light-transmissive photosensitive film is moved through an exposure area in intimate contact with an original document to differentially desensitize the photosensitive film according to the differential reflectivity of the original document. The exposure control continuously collects a plurality of samples of light representative of the sum of the intensity of the light from the exposure light source and the intensity of the light reflected from the portion of the original document at the exposure area, each sample representing the sum over a portion of the exposure area. The samples of light are simultaneously transmitted, their intensities are averaged and the output from the exposure light source is controlled in response to the average intensity to maintain the intensity of the light which exposes the photosensitive film a predetermined constant.

United States Patent [191 Collins [451 Nov. 27, 1973 l5 Inventor:

[ 1 PHOTOCOPIER EXPOSURE CONTROL John E. Collins, North Hudson, Wis.

[73] Assignee: Minnesota Mining and Manufacturing Company, St. Paul,Minn.

[22] Filed: Aug. 23, 1972 [2i] Appl. No.: 282,973

[52] US. Cl 355/83, 240/1 EL, 350/96 R,

I 355/1, 355/68 [51'] Int. Cl. G03b 27/78 [58] Field of Search 355 /83,68, 69, l; 350/96 R, 96 B [56] References Cited UNITED STATES PATENTS2,572,930 10 1951 Heldens 355/83 3,642,377 2/1972 Young 355/83 3,559,5552/1971 Street 350/96 B 3,554,109 1/1971 Street 350/96 B PrimaryExaminer-Richard'L. Moses v Attorney-Kinney, Alexander, Sell, Steldt &Delahunt [571 ABSTRACT An automatic exposure control for a reflexphotocopier wherein a light-transmissive photosensitive film is movedthrough an exposure area in intimate contact with an original documentto differentially desensitize the photosensitive film according to thedifferential reflectivity of the original document. The exposure controlcontinuously collects a plurality of samples of light representative ofthe sum of the intensity of the light from'the exposure light source andthe intensity of the light reflected from the portion of the originaldocument at the exposure area, each sample representing the sum over aportion of the exposure area. The samples of light are simultaneouslytransmitted, their intensities are averaged and the output from theexposure light source is controlled in response to the average intensityto maintain the intensity of the light which exposes the photosensitivefilm a predetermined constant.

' 9 Claims, 3Drawing Figures PHOTOCOPIER EXPOSURE CONTROL FIELD OF THINVENTION The present invention relates to an automatic exposure controlfor a reflex photocopier to provide proper desensitization of aphotosensitive film with original documents having varying image andnon-image areas,

BACKGROUND OF THE INVENTION A reflex photocopier generally includes alight source, a diffuse reflector, and means for positioning an originaldocument to be copied and alight-transmissive photosensitive film inintimate contactxtherewith. The

, photosensitive film-original document composite may be stationarilypositioned during exposure atan exposure area having dimensions at leastas great as'the composite or the composite may be moved through arelatively short but wide exposure area. In the reflex position thephotosensitive film lies between the printed surface of the originaldocument and the light source. The light passes through thephotosensitive film, being absorbed at the black or colored imaged areasand reflected 'at the less absorptive white backproportional to theproduct of the desensitizing lightintensity and the exposure time. Theexposure is continued for a time just sufficient to completelydesensitize the doubly exposed film above the background areas, the filmover the imaged areas thus retaining a degree of sensitivity orreactivity. Following the exposure and dependent upon the type ofphotosensitive film utilized, the film is heated to develop the imagesthereon or it is placed against a coated receptor sheet and thecomposite is heated to produce the images onthe receptor sheet. I

The reflex exposure cycle must be carefully controlled to completelydesensitize the areas of the photosensitive film over the backgroundareas of the original document while retaining a degree of sensitivityor reactivity in the areas thereof over the imaged areas of the originaldocument. Different production runs of the photosensitive film mayproduce variances in the photosensitivity of the'film. This problemhasbeen realized in the prior art and manual exposure control settings havebeen provided on reflex photocopiers to control the duration of anexposure cycle to adjust for these variances.

Very recently it has been found that during reflex exposure, lightdirected against and then reflected from an original document beingcopied is again reflected from the diffuse reflector onto thephotosensitive film and that the intensity of this reflected light. issufficiently great to vary, according to the reflectivity of theoriginal document, the time required to totally desensitize thephotosensitive film over background areas of the original document.United States application Ser. No. 166,607 filed on July 27, 1971, andassigned to the assignee of this application, discloses an apparatus andmethod for compensating for this variation. That application discloses areflex photocopier in which the photosensitive film-original documentcomposite is stationarily positioned during an exposure cycle. Asampleof light is collected whichis representative of the sum of the intensityof the light from the source and the intensity of light reflected fromthe original document. The

SUMMARY OF THE INVENTION It has now beenfound that in reflexphotocopiers wherein the photosensitive film-original documentcompositeprogressesthrough a relatively short but wide exposure areaduring the exposure cycle, the average of a plurality of samples oflight representative of the intensity of the lightfrom the source andthe intensity of the light reflected from the original document, eachsample representing the sum over a portion of the exposure area, moreadequately determines the light intensity level for properdesensitization of the portion of the photosensitive film in theexposure area. Further, it has been found that more effective control ofthe degree of desensitization is obtained by continuously controllingthe output from the light source with respect to a short exposure areasince the degree of desensitization is varied with respect to thereflectivity of only a short segment of the original document and theoverall effect is to carefully control the exposure cycle with respectto a continuously changing short segment thereby providinga betteraverage desensitization of the photosensitive film.

According to the present invention an automatic exposure control isprovided in combination with a reflex photocopier wherein thephotosensitive film-original document composite is moved through apredetermined exposure area at a constant velocity. The exposure controlcomprises means for collecting at a plurality of predetermined spacedpositions continuously during the exposure cycle samples of lightrepresentative of the sum of the intensity of the light from the sourceand the intensity of the light reflected from the original document atthe exposure area, each of the samples representing said'sum over aportion of the'exposure area; means for averaging the intensities of thesamples of light; means for simultaneously transmitting samples of lightfrom the means for collecting to the means for averaging; and meansresponsive to the means for averaging for controlling the output fromthe light source to maintain the sum of the intensity of the light fromthe source from the intensity of the light reflected from the originaldocument at the exposure area a predetermined constant.

THE DRAWING In the drawing:

FIG. 1 is a side elevation view of a photocopier embodying the automaticexposure control of the present invention and having the side framethereof removed;

FIG. 2 is a cross-sectional view taken generally along line 2-2 of FIG.1; and

FIG. 3 is a schematic circuit diagram of the control circuit of theautomatic exposure control of the present invention.

The photocopier, generally designated 10, includes a support frame 12,an elongate tubular exposure lamp 14 supported by the frame 12, and aglass cylinder 16 supported by the frame 12 coaxially with the lamp 14for free rotation about its axis. The glass cylinder 16 is internallyfrosted with a white paint to make it translu- I cent so, that it willdiffuse light from the lamp 14 asthe light passes through the cylinderwall."

Three guide rollers 18, 19 and 20 are supported by the frame l2for freerotation about axes parallel to the axis of the exposure cylinder 16 todefine a drive belt" guide path having an angle of wrap around theexposure cylinder 16 of approximately 240. An endless drive belt 22extends along the belt guide path around the exposure cylinder 16. Aconstant speed motor (not shown) drives guide roller 20 clockwise asviewed in FIG. 1 to provide constant speed drive of the drive belt 22thereby with the cylinder 16 and the drive belt 22 to provide means formoving a light-transmissive photosensitive sheet 21 and an originaldocument 23 in intimate contact therewith through a predeterminedexposure area around the lamp 14 at a constant velocity. The photocopierframe 12 is formed with an opening in the front thereof leading into anentrance chute 24 for guiding a light-transmissive photosensitive sheetand an original document in intimate contact therewith into the initialcontact between theexposure cylinder 16 and the drive belt 22. And, anexit ramp 27 defines a guide path for the photosensitive sheet-originaldocument combination from the departing contact of the exposure cylinder16 and the drive belt 22 to the opening in the front of the machine. Askiving knife 28 is supported by the exit ramp 27 to skive aphotosensitive sheet-original document combination off the exposurecylinderv16 and onto the exit ramp 27. I

Afiber optic bundle support bar 31 is supported by the frame 12 toextend parallel to the axis of the exposure cylinder.l6 in the areabetween the entrance chute 24 and the exit ramp 27. A plurality of fiberoptic bundles 33 pass through apertures in the support bar 31 at equallyspaced intervals along the axis of the exposure cylinder 16 to positionone end of each of the fiber optic bundles 33 adjacent the exposurecylinder 16.

The fiber optic bundles 33 are spaced axially along the cylinder 16 adistance generally corresponding to the width of the light-transmissivephotosensitive sheet to each bundle provides a portion of the lightreceived by.

the photocell 35. The photocell 3S sums the intensities of the lightcollected by the fiber optic bundles 33 and, therefore, it provides ameans for averaging the intensities of the light at the locations atwhich the fiber optic bundles 33 collect exposure light.

An angle bar 37 is supported by the frame 12 within the exposurecylinder 16 parallel to the lamp 14 along the entire length thereof andbetween the lamp 14 and the ends of the fiber optic bundles 33 adjacentthe cylinder 16 to shield the fiber optic bundles 33 from direct lightfrom the lamp 14. Thus, the fiber optic bundles 33 collect only a lightfrom the lamp l4which has at least been once reflected.

The light diffusing property of the cylinder 16 causes uniform diffusionof the light from the lamp l4 and thereby in effect acts as a diffusereflector for reflecting light during an exposure cycle from the lamp l4uniformly onto a predetermined exposure area at the periphery of theexposure cylinder 16 extending counterclockwise as viewed in FIG. 1 fromtne entrance chute 24 to the skiving knife 28. The fiber optic bundles33 4 collect light reflected from the internal surface of the cylinder16 and light reflected from the background areas of an original document23 moved between the exposure cylinder 16 and the drive belt 22 inintimate contact with a light-transmissive photosensitive sheet 21,thephotosensitive film lying between the original document and'the'lamp14. Each'fiber optic bundle is sensitive to the intensity of the lightacross a small width of the original document and, thereby," the sum ofthe intensities collected by the fiber optic bundles 33 provides anaverage across the width of the original document 23 of the sum of theintensity of the light reflected from the interior of the cylinder andthe intensity of the light reflected from the portion of the originaldocument in the exposure area.

The control circuit generally designated 50, shown in FIG. 3, serves tovary" the light output from the lamp 14 to cause the reflected lightdetected by the fiber optic bundles 33 to remain at a predeterminedconstant level. The light collected by the plurality of fiber opticbundles 33, which is reflected from the inside of the exposure cylinder16 and the document being copied, is piped by the fiber optic bundles tothe photocell 35. These optical paths are indicated in FIG. 3 by thesingle dotted line 52. With this arrangement, the resistance of thephotocell 35 is a measure of the total reflected light energy detectedby the fiber optic bundles 33 at any instant of time. v V

The resistivity of the photocell 35 is used in the circuitry of FIG. 3to develop a signal for controlling the conduction of a controlledbidirectional semiconductor switching device 54 connected in series withthe lamp 14 across an AC power source. An inductor 56, which merelyserves to Suppress any radio frequency signals that may; be developed bythe on-off operation of the switching device 54, is also connected inseries with the device 54 and lamp 14. In the embodiment shown, thedevice 54 is a triac. The circuit is arranged to decrease the time thatthe triac S4 conducts during each cycle of AC power when the lightenergy detected by the photocell 35 increases above a desired level andto increase the conducting time when the light energy detected dropsbelow a desired level.

A capacitor 58 is shown connected across the triac 54 which serves todelay the application .of full voltage to the triac 54. If capacitor 54were not used, it might be possible for the triac 54 to be turned onwhen the AC power is initially applied to the circuit.

The circuitry shown to the right of triac 54 develops the triggeringsignal for the triac. The triggering signal circuit is connected acrossthe triac 54 and includes a power supply portion. The power supplyportion includes diodes 41-44 arranged in conventional fashion toprovide full wave rectification of the AC power presented across thetriac 54. When conducting, the triac 54 presents a virtual short acrossthe triggering signal circuitry, so the power supply portion of thetriggering signal circuitry will only supply a DC voltage when the triac54 is not conducting. A DC triggering pulse can be provided for eachhalf cycle of the AC power so the triac 54 can conduct for at least aportion of each half cycle of the AC power.

The DC power supply portion of the circuitry also in- 62. A smallcapacitor 64, which merely servesto elimi- NPN type transistors 66 and68.1 The photocell 35 is connected in the base electrode circuitry forthe transistor 66.Changes in the resistivityof the photocell 35 alterthe base current of the transistor 66and therefore the conduction oftransistor 66. The base current of the transistor 68 is determinedprimarily by the series connected resistors 70, 72 and 74. Theresistor72 is the resistive portion of a, potentiometer 76 which has itswiper or movable contact 78 connected to the base electrode oftransistor 68. The potentiometer 76 thus provides the means forselecting the set point or reference voltage for the differentialamplifier and therefore the light level to be maintained. The output ofthe differential amplifier is-obtained at the collector of transistor68.This outputvaries in accordancewith the difference between the basecurrent of transistor 68 and the base current of transistor 66. Sincethe setting of potentiometer 76 establishes the average light level thatis to be maintained for the photographic or copying process, it isplacedin a position where it cTn be adjusted by the operator. I

The base electrode circuitry for transistor 66 ineludes the seriescircuit comprising a Zener diode 80,

66 when the resistance of photocell 35 is initially very high prior tothe lamp -l4.being turned on. The emitter electrodes of transistors 66and 68 are each connected to a resistor 90 which connects to aconductor;92. The

} conductor 92 is a common conductor which onnects to the anode side ofZener diode 62, one side 0 capacitors 70, 72 and 74 is connected to theconductor 92 while resistor 70 is connected to the conductor 96 via thecollector and emitter electrodes of a transistor 98. The transistor 98is a PNP type transistor so the emitter electrode is connected to thecommon conductor 96 which is positive with respect to the conductor 92.The resistor 70 is shown to be variable and is used to compensate fortolerances in the other components determining the base current oftransistor 68. i

The output of the differential amplifier, which refleets the changes inthe level of light energy applied to the photocell 35, is applied to anNPN transistor 100 e which has its base electrode connected to thecollector of transistor 68 of the differential amplifier. The emitter oftransistorj l00 is connected to the common conductor 96 via a resistor102 while its collector electrode is connected to'one side of acapacitor 104. The other side of capacitor 104 is connected to thecommon conductor 92.

The emitter electrode of a unijunction transistor 106 is connectedto theconnection 108common' to the col- .lectorelectrode of transistor andcapacitor 104. The base-one electrode of the unijunction transistor 106is connected'tothe common conductor 96 via a resistor 109 while thebase-two electrode is connected to the common conductor 92 viathe-primary winding 110 of a pulse transformer. Thesecondary winding 112of the pulse transformer is connected between the gate electrode oftriac 54 and one side of the AC power supp y- I Transistorl00 serves asa current source, which when conducting, causes capacitor 104 to becharged to a voltage level sufficient to tire the unijunction transistor106. Conduction of transistor 106 causes the capacitor 104 to be rapidlydischarged via the primary winding 110 of a pulse transformer inducing apulse at the secondary winding 112 connected to the gate electrode ofthe triac 54 causing the triac to be turned on.

In order to make certain that the voltage on capacitor 104 will alwaysreach a value sufficient to fire the unijunction transistor 106, thebase electrode of transistor 100 is connected via a diode 114 to theconnection 1 16 that is common to the photocell 35 and Zener diode 80.With this arrangement the voltage at base of the transistor 100 willalways be at least approximately equal to the voltage provided by the.Zener diode 80. The Zener diode 80 istherefore selected to provide avoltage which is greater than the voltage needed to trigger theunijunction-transistor'106. 1

t The transistor 98 connected to the differential amplifier permitsconduction of transistor 68 of the differential amplifier to graduallyincrease for several cycles of the AC power when the circuit isinitiallyv energized so that full power isnot immediately applied to thelamp 14. This serves to extend the life of the lamp 14. Conduction oftransistor 68 increases during a time delay of approximately 300milliseconds provided by the RC circuit connected to transistor 98 whichincludes ca-' pacitor 118 and resistor 120. The RC circuit allows thevoltage at the base of transistor 98 needed for conduction to increasegradually. The capacitor 118 is connected in series with resistor 120and has one side connected to the common conductor 96. The resistor 120isconnected to the common conductor 92 via a diode 122. The baseelectrode of transistor 98 is connected via resistor 124 to theconnection 126 that is common to capacitor 118 and resistor 120. Adischarge path for the capacitor 118 is provided by a resistor 128connected in parallel with capacitor 118.

In use, the constant speed belt drive motor (not shown) isenergized tomove the drive belt 22 at constant speed and the circuitry shown in FIG.3 is placed in operation by closure of the switch 130 which connects therectifying diodes 41 and 42 to the connection 132 common to triac 54 andinductor 56. Until the switch 130 is closed, a triggering signal cannotbe produced and applied to the triac 54 to cause triac 54 to conduct toenergize the lamp 14. Since the lamp 14 is initially not energized, theresistance presented by the photocell 35 is high. This being the case,conduction of transistor 68 upon closureof switch 130 will increase 7 7gradually during the 300 millisecond time delay introduced by transistor98 and its associated circuitry. Since the resistance presented by thephotocell 35 is initially high, conduction of the transistor 68 willreach a level sufficient to reduce the voltage at the collector oftransistor 68 to the level provided by the Zener diode 80 and diode 114causing the transistor 100 to conduct at a high level to quickly chargecapacitor 104. Capacitor 104 will charge to a voltage sufficient to fireunijunction transistor 106 causing the capacitor 104 to be dischargedvia the primary winding 110 of the pulse transformer to produce atriggering signal at the secondary winding. 112 connected to the gate oftriac 54. As the level of conduction of transistor 68 increases,

'the time for charging capacitor 104 decreases so the portion of eachcycle when the triac 54 conducts is gradually increased over a number ofcycles of the AC power due to the time delay provided by transistor 98and its associated circuitry. About one second is required to dischargecapacitor 118 so no further delay is presented to the differentialamplifier circuit by the transistor 98 circuitry following the initialoperation of the circuitry.

Since the DC power supply circuitry for the ing circuit providesfullwave rectification of the AC power, a pulse of DC voltage ispresented to the triggering circuitfor each half cycle of the AC powercausing the triggering circuit to provide a triggering signal to firethe triac 54 for eachhalf cycle.

Exposure of a photosensitive sheet 21 is initiated by placing it inintimate contact with an original document 23 and sliding thecombination down the entrance chute 24 with the original document on thebottom. The leading edge of the combination enters between the drivebelt 22 and the exposure cylinder 16 and the combination is, pulledthrough the exposure area around the lamp 14 until it passes over theskiving knife 28 and out of the exit ramp 27. As the photosensitivefilm-original document combination progresses through the exposure areathe firing of the triac 54 continues and the photocell 35 receivestriggerthe light energy detected by the fiber optic bundles 33.

Light received by the photocell 35 causes the resistance of thephotocell to drop from its initial high value and to vary in accordancewith the light energy received via the fiber optics bundles representedin FIG. 3 by the dotted line 52. Once the light 14 is energized, thevoltage at the collector of transistor 68 will rise above the minimumlevel provided by the Zener diode 80 and diode 114. Since voltage at thecollector of transistor 68 is then determined by the setting of thepotentiometer 76 and the resistivity of the photocell 35, theunijunction transistor 106 will fire at the same point during each halfwave of the AC power so long as the light energy received by thephotocell 35 remains constant. Should the conductivity of the photocell35 increase due to an increase in the light energy received, transistor66 will conduct more'causing transistor 68 to conduct less. A reductionin the collector current of transistor 68 causes the voltage at thecollector of transistor 68 to increase to reduce the level of conductionfor transistor 100. A reduction in the conduction of transistor 100increases the time required for capacitor 104 to charge to a levelsufficient to fire the unijunction transistor 106. A triggering signalwill thus be provided the triac 54 at a point later in the half cycle ofthe AC power then being presented causing a reduction in the currentflow through the lamp 14 to thus reduce the light energy level. Areduction in the light energy level detected by the fiber optic bundles33 which is applied to the photocell 35 will cause the conductivity ofthe photocell '35 to decrease which in turncauses a reduction in thecurrent flow through'transistor 66 causing the transistor 68 to conductat a higher level to reduce the voltage at the collector of transistor68. This causes the transistor 100 to conduct at a higher level causingthe capacitor 104 to charge more rapidlyto a level sufficient to firethe unijunction transistor 106. A triggering signal is thus presented toincrease the current flow through the lamp l4 and increase thelightenergy level. In this manner, the light output of lamp 14 is controlledso the light energy level detected by the fiber optic bundles 33 andtransmitted to the photocell '35 in accordance with the selected settingof the potentiometer 76 will be maintained constant.

When the entire length of the photosensitive sheetoriginal documentcombination has progressed through the exposure area it is removed fromthe exit ramp 27. Depending upon the type of photosensitive film beingused, the photosensitive sheet is then heated alone to develop theimages thereon or it is placed in contact with a coated receptor sheetand the composite is heated to produce the images on the receptor sheet.

A specific example of a control circuit 50 can be constructed as shownin FIG. 3 using transistors having at least a 25 volt breakdown ratingwith abe ta of 80 or more and 5 photocell rated at 308fiand 2footcandles.

The triac 54 can be any having a rating of at least 25 Amperes, 220volts. Typical values or types for other components shown in FIG. 3 arelisted below:

Lamp V, 1000 W Zener Diode 62 10 V Zener Diode 80 4.7 V

1. In a photocopier including a light source, a diffuser reflector forreflecting light during an exposure cycle from the source uniformly ontoa predetermined exposure area and means for moving a light-transmissivephotosensitive film and an original document in intimate contacttherewith through the predetermined exposure area at a constantvelocity, the original docu- -ment having image and non-image areasproviding a differential reflectivity, the photosensitive film beingbetween the source of exposure light and the original document and thephotosensitive film being differentially desensitized according to thedifferential reflectivity of the original document, an automaticexposure control comprising:

means for collecting at a plurality of predetermined spaced positionscontinuously during the exposure cycle samples of light representativeof the sum of the intensity of the light from the source and theintensity of the light reflected from, the original document at theexposure area, each said sample representing said sum over a portion ofthe exposure area, means for averaging the intensities of a saidplurality of samples of light, means for simultaneously transmittingsamples of light from said means for collecting to said means foraveraging, and means responsive to said means for averaging forcontrolling the output from the light source to maintain the sum of theintensity of the light from the source and the intensity of the lightreflected from the original document a predetermined constant at theexposure area, said means for controlling including a semi-conductorswitching device connected in series with the light source and a signaltriggering circuit for said device, said triggering circuit beingconnected to said means for averaging for control thereby. A 2. Thephotocopier of claim 1 wherein said means for collecting and said meansfor transmitting comprise a plurality of fiber optic bundles, each saidfiber optic bundle having one end at one of said predeterminedcontrolling the light source includes a semi-conductor switching deviceconnected in series with the light source and a signal triggeringcircuit for said device, said triggering circuit connected to said meansfor averaging for control thereby.

5. The photocopier of claim 1 wherein said signal triggering circuitincludes a pulse producing circuit portion for applying a triggeringpulse to said semiconductor switching device, a differential amplifierconnected to said means for averaging and said pulse producing circuitfor controlling said pulse producing circuit in accordance with saidmeans for averaging.

6. The photocopier of claim 5 wherein said signal triggering circuitincludes means connected to said differential amplifier for determiningthe set point for said signal triggering circuit.

7. The photocopier of claim 5 wherein said signal triggering circuitincludes means limiting the operation of said differential amplifierwhen said signal triggering circuit is initially energized.

8. The photocopier of claim 5 wherein said pulse producing circuitincludes a pulse transformer coupled to spaced positions and itsopposite end connected to' said 7 said semi-conductor switching deviceand a capacitor controlled unijunction transistor connected to saidpulse transformer.

' 9. The photocopier of claim 1 wherein the photocopier is an a.c.operated copier, said semi-conductor switching device is connected inseries with the light source for energization by an a.c. power sourceand said signal triggering circuit provides a triggering signal for saiddevice during each half cycle of the a.c. power source.

1. In a photocopier including a light source, a diffuser reflector forreflecting light during an exposure cycle from the source uniformly ontoa predetermined exposure area and means for moving a light-transmissivephotosensitive film and an original document in intimate contacttherewith through the predetermined exposure area at a constantvelocity, the original document having image and non-image areasproviding a differential reflectivity, the photosensitive film beingbetween the source of exposure light and the original document and thephotosensitive film being differentially desensitized according to thedifferential reflectivity of the original document, an automaticexposure control comprising: means for collecting at a plurality ofpredetermined spaced positions continuously during the exposure cyclesamples of light representative of the sum of the intensity of the lightfrom the source and the intensity of the light reflected from theoriginal document at the exposure area, each said sample representingsaid sum over a portion of the exposure area, means for averaging theintensities of a said plurality of samples of light, means forsimultaneously transmitting samples of light from said means forcollecting to said means for averaging, and means responsive to saidmeans for averaging for controlling the output from the light source tomaintain the sum of the intensity of the light from the source and theintensity of the light reflected from the original document apredetermined constant at the exposure area, said means for controllingincluding a semi-conductor switching device connected in series with thelight source and a signal triggering circuit for said device, saidtriggering circuit being connected to said means for averaging forcontrol thereby.
 2. The photocopier of claim 1 wherein said means forcollecting and said means for transmitting comprise a plurality of fiberoptic bundles, each said fiber optic bundle having one end at one ofsaid predetermined spaced positions and its opposite end connected tosaid means for averaging.
 3. The photocopier of claim 2 wherein saidmeans for averaging comprises a single photocell positioned to receivethe plurality of samples of light collected and transmitted by saidfiber optic bundles.
 4. The photocopier of claim 1 wherein said meansfor controlling the light source includes a semi-conductor switchingdevice connected in series with the light source and a signal triggeringcircuit for said device, said triggering circuit connected to said meansfor averaging for control thereby.
 5. The photocopier of claim 1 whereinsaid signal triggering circuit includes a pulse producing circuitportion for applying a triggering pulse to said semi-conductor switchingdevice, a differential amplifier connected to said means for averagingand said pulse producing circuit for controlling said pulse producingcircuit in accordance with said means for averaging.
 6. The photocopierof claim 5 wherein said signal triggering circuit includes meansconnected to said differential amplifier for determining the set pointfor said signal triggering circuit.
 7. The photocopier of claim 5wherein said signal triggering circuit includes means limiting theoperation of said differential amplifier when said signal triggeringcircuit is initially energized.
 8. The photocopier of clAim 5 whereinsaid pulse producing circuit includes a pulse transformer coupled tosaid semi-conductor switching device and a capacitor controlledunijunction transistor connected to said pulse transformer.
 9. Thephotocopier of claim 1 wherein the photocopier is an a.c. operatedcopier, said semi-conductor switching device is connected in series withthe light source for energization by an a.c. power source and saidsignal triggering circuit provides a triggering signal for said deviceduring each half cycle of the a.c. power source.